1. Field of the Invention
The present invention relates to an image (or video) signal processing apparatus for image display which is suitable for an image pickup apparatus such as a digital camera or the like.
2. Related Background Art
FIG. 27 is a block diagram showing a schematic structure of a conventional image pickup apparatus. In FIG. 27, an image pickup element 1010 converts an optical image into an image signal, and an A/D (analog-to-digital) converter 1012 converts the analog output of the image pickup element 1010 into a digital signal. An image pickup signal processing circuit 1014 creates a brightness signal by performing color carrier elimination, aperture correction, a gamma process and the like to the output data of the A/D converter 1012, and simultaneously creates a color difference signal by performing color interpolation, matrix conversion, a gamma process, gain adjustment and the like to the output data of the A/D converter 1012, whereby image data of YUV format is created.
A memory I/F (interface) 1016 which includes writing and reading circuits 1016a and 1016b for a memory 1018, writes the image data from the image pickup signal processing circuit 1014 in the memory 1018, reads image data from the memory 1018, and outputs the read image data to a display system signal processing circuit 1020.
The display system signal processing circuit 1020 separates from the image data of YUV format a brightness component Y and a modified color difference component, i.e., a modified chroma component signal C, and then outputs them respectively to D/A (digital-to-analog) converters 1022Y and 1022C. The D/A converter 1022Y converts the brightness data from the display system signal processing circuit 1020 into an analog signal, and an LPF (low-pass filter) 1024Y eliminates a high-frequency noise component from the output of the D/A converter 1022Y. The output signal from the LPF 1024Y is transferred to a mixture circuit 1026 and an LCD (liquid crystal display) control circuit 1028. The D/A converter 1022C converts the modified chroma data from the display system signal processing circuit 1020 into an analog signal, and a BPF (band-pass filter) 1024C extracts only a frequency component of a modified chroma component from the output of the D/A converter 1022C. The output signal from the BPF 1024C is transferred to the mixture circuit 1026 and the LCD control circuit 1028.
The mixture circuit 1026 adds the brightness signal from the LPF 1024Y and the modified chroma signal from the BPF 1024C together to create a composite video signal. A video amplifier 1030 amplifies the composite video signal output from the mixture circuit 1026, and transfers the amplified signal to a TV monitor 1032, whereby the image obtained by the image pickup element 1010 is displayed on a screen of the TV monitor 1032.
The LCD control circuit 1028 converts the brightness signal Y from the LPF 1024Y and the modified chroma signal C from the BPF 1024C into an RGB signal composed of R (red), G (green) and B (blue) components, in accordance with a subcarrier frequency from a quartz oscillator 1034. The RGB signal is transferred to an LCD panel 1036 together with a driving pulse, whereby the image obtained by the image pickup element 1010 is displayed on a screen of the LCD panel 1036.
FIG. 28 is a block diagram showing a schematic structure of another conventional image pickup apparatus. In FIG. 28, operations of an image pickup element 1110, an A/D converter 1112, an image pickup signal processing circuit 1114, a memory I/F 1116, a writing circuit 1116a, a reading circuit 1116b and a memory 1118 are respectively the same of the operations of the image pickup element 1010, the A/D converter 1012, the image pickup signal processing circuit 1014, the memory I/F 1016, the writing circuit 1016a, the reading circuit 1016b and the memory 1018, respectively. Thus, a YUV-system image data is similarly transferred to a display-system signal processing circuit 1120.
The display-system signal processing circuit 1120 creates and outputs, from the YUV-system image data, a brightness signal Y, a composite video signal CV by adding the brightness component and a modified chroma signal C, a color difference signal U (=B−Y) by eliminating the brightness component from the B component, and a color difference signal V (=R−Y) by eliminating the brightness component from the R component.
A D/A converter 1122a converts the composite video data from the display-system signal processing circuit 1120 into an analog signal, and an LPF 1124a eliminates a high-frequency noise component from the output of the D/A converter 1122a. A video amplifier 1126 amplifies the output of the LPF 1124a and transfers the amplified signal to a TV monitor 1128.
D/A converters 1122b, 1122c and 1122d convert the brightness data Y, the color difference data U and the color difference data V from the display-system signal processing circuit 1120 into analog signals, respectively. LPF's 1124b, 1124c and 1124d eliminate high-frequency noise components from the outputs of the D/A converters 1122b, 1122c and 1122d, respectively. The outputs from the LPF's 1124b, 1124c and 1124d are transferred to an LCD control circuit 1130. It should be noted that each of the LPF's 1124c and 1124d ordinarily has a pass band of 1.5 MHz to 2.0 MHz.
The LCD control circuit 1130 creates an RGB signal from the brightness signal Y from the LPF 1124b, the color difference signal U from the LPF 1124c, and the color difference signal V from the LPF 1124d. The RGB signal is transferred to an LCD panel 1132 together with a driving pulse, whereby the image obtained by the image pickup element 1110 is displayed on a screen of the LCD panel 1132.
However, in the conventional image pickup apparatus shown in FIG. 27, since the RGB signal is created from the modified chroma signal C, the quartz oscillator 1034 generating subcarrier frequency is necessary, whereby the number of parts is increased. Further, in order to deal with plural different image signal systems, the circuit structure must be made different according to each image signal system.
As color television image signal systems, as well known, there are an NTSC (National Television System Committee) system adopted in Japan, United States and the like, and a PAL (Phase Alternation by Line color television) system adopted in European nations and the like. Since a carrier frequency of subcarrier is about 3.58 MHz in the NTSC system and about 4.43 MHz in the PAL system, the same crystal oscillator can not be used for the NTSC system and the PAL system. As a result, the circuit structure must be made different between these two image signal systems, whereby the number of parts is increased. Further, different adjustment is necessary for each of these different circuit structures, whereby production efficiency (mass productivity) decreases.
Further, in the conventional image pickup apparatus shown in FIG. 28, since the RGB signal is created from the brightness signal Y and the two color difference signals U and V, any quartz oscillator for generating subcarrier frequency is not necessary, whereby the number of parts can be decreased. Further, the structure of the LCD control circuit 1130 can be made the same irrespective of the image signal system. Therefore, this image pickup apparatus is excellent in the point of mass productivity as compared with the conventional image pickup apparatus shown in FIG. 27. Further, the image pickup apparatus shown in FIG. 28 can achieve high image quality.
However, in the conventional image pickup apparatus shown in FIG. 28, as described above, since the RGB signal is created from the brightness signal Y and the color difference signals U and V, the three D/A converters 1122b, 1122c and 1122d are necessary for the liquid crystal display operation. Further, in order to simultaneously display the image on the TV monitor and the LCD panel, the four kinds of signals CV, Y, U and V must be output in total, whereby the four D/A converters 1122a, 1122b, 1122c and 1122d are necessary in this case. When ASIC (application specific IC (integrated circuit)) is achieved with three or four D/A converters, there is a problem that the dimensions of circuits become large.
Further, if circuit miniaturization or downsizing due to the ASIC advances, since a power supply voltage of the ASIC core becomes lower, the circuit dimensions of the D/A converter to which a predetermined-level analog output is always necessary become relatively large. Thus, it is actually difficult to achieve ASIC's with many D/A converters.